The present invention relates to a process for the preparation of amorphous polycarbonate polyols.
Various processes for the preparation of polycarbonate polyols are known in the art. For example, U.S. Pat. No. 3,379,693 discloses a process for producing carbonate compositions which comprises heating at least one cyclic carbonate with at least one organic functional initiator (e.g., an alkylene diol) to a temperature in the range of about 50.degree. to 225.degree. C. The reaction is essentially a homopolymerization of the cyclic carbonate. The function of the diol or other initiator is to open the ring of the cyclic carbonate so as to provide active sites with which other cyclic carbonate molecules are capable of reacting. It is apparent that as the concentration of initiator in the reaction system is increased, the number of initiated polymer chains also increases, thereby reducing the molecular weight for a given amount of cyclic carbonate. Therefore, for a given level of diol initiator, the molecular weight of the product is fixed within a narrow range. There is therefore very little flexibility for varying both molecular weight and the amount of alkylene diol which can be incorporated into the polymer.
Other known processes involve the transesterification of aliphatic dihydroxy compounds with glycol carbonates, as illustrated by the processes disclosed in U.S. Pat. Nos. 4,105,641, 4,131,731, etc. Such transesterification reactions require the removal of by-product glycol. In contrast, the process of the present invention involves a simple esterification technique.
U.S. Pat. No. 4,365,055 discloses the preparation of a substantially linear carbonate polymer wherein carbonic dihalide is introduced to a substantially anhydrous solution comprising at least one substituted or unsubstituted 1,3-propanediol, the reaction being conducted in the presence of a nitrogen-containing organic catalyst, such as pyridine. While it is stated that the coproduction of cyclic carbonate is reduced by the presence of the catalyst during the reaction, the amount of cyclic carbonate produced is still undesirably high. In contrast, the process of the present invention can be employed to produce a specified class of polycarbonate polyols with the coproduction of only minimal amounts of cyclic carbonate.